The present invention concerns the generating of shocks in the ground for seismic exploration, and in particular concerns the production of shocks by the dropping of a weight on to the ground.
The weight dropping method has been known for a long time and is particularly described in the U.S. Pat. No. 2,851,121. In this known method a lorry fitted with a lifting jib is used to raise a heavy solid body to a certain height which is then allowed to fall on to the ground. In current practice, for example, a heavy mass of three tons is dropped from a height of three meters. Usually the energy released in the ground by the shock produced by the impact of the heavy mass is relatively low, and so it is advantageous to use a succession of such shocks giving rise to different, refracted or reflected waves which return through the ground to seismic detectors, and to integrate the different signals corresponding to different shocks to obtain a composite signal containing significant data.
The method of prospecting by weight dropping, described above, offers the essential advantage of being able to be carried out near houses (which is not the case with methods involving the exploding of charges); it is not in itself very effective from the seismic point of view, in the sense that it only introduces low amounts of energy into the ground; in any case, it requires the use of a heavy land vehicle, which precludes its use in hilly areas.
It is an object of the present invention to provide a new mode of seismic prospecting by weight dropping particularly involving a new heavy mass.
Another object of the present invention consists in the replacing of the land vehicle previously used by an aircraft, particularly a balloon or a helicopter. Helicopters are known to be more and more used in seismic expeditions. A priori, the use of aircraft allows the release of a heavy mass from a height much greater than usual heights and the potential energy of the heavy mass, which varies with the height of the drop, can be especially increased thereby, all things staying equal in other respects. For example, it seems advantageous, from the energy point of view, to work with a mass of 500 kgs for dropping heights comprised between 5 and 100 meters.
In a complementary way, the increase in the dropping height increasing the speed of arrival on the ground results in an appreciable enlargement of the spectrum of the seismic wave produced, which improves the seismic data obtained.
The first trials conducted by applicants consisted purely and simply in transposing the conventional technique to the release-by-aircraft situation, using a hovering helicopter equipped for transporting by sling and fitted with a system of electromagnetic release controlled by the pilot. Heavy masses, solid and rigid, of various shapes were released thus from various heights.
The results obtained proved nearly unusable for all of the following reasons:
on hard terrain and even on relatively loose terrain, the mass rebounds on its first impact with the ground and gives rise to a series of successive shocks in a random and unpredictable way; for that reason, the reflected or refracted seismic waves are not exploitable in practice: they contain parasite waves.
on very loose terrain, the mass buries itself in the ground, and there is a barely usable shock wave, and a significant deterioration in the surface of the ground.
Thus, it is apparent that the method putting into practice a weight drop from an aircraft cannot be a simple transposition of known weight dropping methods.
Essentially, for a seismic prospecting by aircraft, the applicants propose a new releasable heavy mass characterised by its structure and by its functions.